This invention relates to an improved flow cell for an optical measuring system. The invention is particularly suited for use in connection with the infrared measurement of liquid dairy products and other liquid food products.
Optical sensing is a common non-invasive manner to measure the various constituents which make-up food products. One effective technique of optical sensing is performed using an infrared light source, fiber optic cables and photodetectors. Infrared light is passed through a food sample and light transmitted through the sample is measured and analyzed by photodetectors. The data generated by the photodetectors can provide an accurate measurement of a given sample's constituent make-up because each component has unique absorbance properties and thus a unique optical signature in the infrared range.
One application for this technology involves optical sensing in connection with the production and processing of dairy products. In the production of dairy products it is desirable to quantitatively measure various constituents of the product such as fat, water, solids and sugar content from a stream of flowing liquid. Measurement of these components involves passing the flowing liquid stream through a light beam emitted from an infrared source and then measuring the light transmitted through the sample with photodetectors. The apparatus which provides a facility to direct an infrared light beam through a flowing sample stream is referred to as a flow cell. Light generated from an infrared source is transmitted through a fiber optic cable to a window in the flow cell which is transparent to infrared light. Infrared light passes through the window and then through the sample stream passing through the flow cell. Light that is not absorbed by the sample then passes through an opposite window to be received by a second fiber optic cable. The second cable transmits the incident light to a photodetector where the intensity of the transmitted light is measured. The various constituents of food products such as milk, salad dressing, cheese, and yogurt have unique absorbance spectra in the infrared range. Quantitative measurements of the constituents of the food products can be by carried out measuring the light transmitted through the sample at predetermined wavelengths in the near infrared range. Using a device as described herein, a food producer can continuously monitor the various levels of constituents in his product throughout the production phase.
In prior art systems, fiber optic cables transmitting infrared light are received in a flow cell by opposite cylindrical extensions positioned perpendicular to an axis parallel to the direction of flow of the sample. Fiber optic cables are received in cavities of closed ended tubes which define distal circular window areas. In the prior art flow cells, the closed ended tubes are made of polysulphone. The tubes radially extend from the sidewalls of the flow cell and into the conduit carrying the food product to position the windows opposite one another. The windows are positioned in this manner so that light only passes through a reduced sample section as compared to the diameter of the conduit. Such a reduced section is generally required because an adequate amount of infrared light cannot sufficiently penetrate a long distance through a sample to enable a photodetector to make accurate measurements. The optimal distance between the opposite windows in a flow cell is dependent on the product that will be measured.
Standard operating practice in the dairy industry dictates that the conduits carrying food products and all the fittings used therein be thoroughly cleaned on a daily basis. Because flow cells are within the conduits and are in contact with the food products they must also be disassembled and cleaned after each use. Generally accepted design parameters for conduits designed to carry dairy products attempt to keep the interior surface of the conduits as smooth and even as possible. Any interruption of the interior surface, such as a crevice, provides a harbor for food products to accumulate, coagulate and spoil. The presence of coagulated milk or dairy products within a conduit provides a suitable environment for the growth of harmful bacteria which can contaminate food product passing through a particular conduit. Moreover, an accumulation can be abruptly released into the food product further contaminating the supply.
The use of closed ended tubes to receive the fiber optic cables were effective in eliminating any crevices or gaps exposed to the flow path at the distal ends of the polysulfone tubes because these tubes were molded of one piece. However, the closed ended tube structure meant that the infrared light had to be passed through polysulfone windows at the distal end of tubes and while polysulfone is transmissive of infrared light, it nevertheless provides significant attenuation of the near infrared light. In addition, the polysulfone is susceptible to cracking when subjected to the high pressure that occurs in the fluid in the flow cell.
In a flow cell, it is sometimes necessary or desirable to adjust the path length in the material through which the infrared light travels to account for liquids with different degrees of absorption. In a prior art flow cell, this adjustment was carried out by changing the length of the closed ended tubes which housed the fiber optic cables and a path length adjustment required a partial disassembly of the flow cell to change to a different length closed ended polysulfone tube.